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Antibiotics: What Are They?

The word antibiotics comes from ancient Greek anti (against) and bios (life). These medicines kill bacteria, the microscopic creatures (germs) that cause infections llike strep throat, pneumonia and many others. The first antibiotic was penicillin, a chemical that is found in a particular kind of mold. Penicillin revolutionized medicine because it allowed doctors to treat the infections that made even minor wounds a potentially life-threatening situation. Since the discovery of penicillin, many other antibiotics have been isolated – all work by killing the bacteria that make us ill, without harming us.

About Clinical Trials

When doing a study of this kind, it is important to make sure that the two groups of people, those who got a disease and the controls, are similar in every measurable way before developing a disease. Failure to do so can lead to wrong conclusions about those factors that increase one’s risk of disease.

For example, if tall women were more likely to take antibiotics than short women and being tall alone increased one’s risk of breast cancer, then failure to make sure that there were equal proportions of tall and short women in the two groups might lead one to falsely conclude that antibiotics cause breast cancer. By carefully looking at the medical records, the authors concluded that the two groups of patients were roughly similar in age, body weight, number of children, and other factors that might affect rates of breast cancer. For characteristics in which the groups were slightly different, researchers made some statistical adjustments to compensate for the differences. As the authors noted, there may have been unknown differences in the two groups that influenced the results.

Resistance to Antibiotics

There is another reason to avoid unnecessary use of antibiotics. This is because occasionally bacteria can acquire new genes that make them resistant to a given kind of antibiotic. The widespread use of antibiotics tends to favor the appearance of these resistant bacteria. People infected with these new germs are very much at risk, as available medicines may be useless in treating their illness. The patient who is prudently limiting antibiotic use for personal health reasons is contributing to health around the world.

Cancer.gov’s Breast Cancer Page
What You Need to Know About Breast Cancer
Understanding Cancer Statistics
MedLine Plus Breast Cancer
What Is Breast Cancer
Breast Cancer Risk Assessment Tool
Breast Cancer Basics (National Alliance of Breast Cancer Oganizations)
MedLinePlus: Antiobiotics
Antiobiotics: When They Can and Can’t Help
Antibiotic Resistance
Antiobiotics Attack
Clinical Trials
Antibiotics and Breast Cancer: Is There Really a Cause for Concern?

Breast cancer is the most frequent cancer among women and a leading cause of premature death. More than 200,000 American women will receive a diagnosis of breast cancer this year. The good news is that breast cancer is usually curable if it is detected early by a woman’s self exam or by a visit to a clinic to obtain a breast exam and mammogram.

The study indicated that extensive antibiotic use was associated with a doubling in the rate of breast cancer compared with women who used no antibiotics at all. This means that rather than having one in eight odds of developing breast cancer over a lifetime, those who used antibiotics for prolonged periods would have odds of one in four.

Interestingly, it didn’t seem to make any difference which antibiotic was used even though different antibiotics have very dissimilar effects on the body.

Does This Mean That Antibiotics Cause Breast Cancer?  

The study does not prove that antibiotics cause breast cancer. It does offer strong evidence that antibiotics are associated with the disease. This is an important distinction! The authors of the study reviewed the medical records of women who had already developed breast cancer. Their finding that these women had taken more antibiotics than the control group might mean that antibiotics cause cancer, but there are other possible explanations. For example, it is possible that some people are less able to resist bacterial infection and that this trait also increases one’s chances of developing breast cancer. These people might well take more antibiotics because they get sick more often. If this were true one might well find that people who had breast cancer had taken more antibiotics, but it would be a mistake to conclude that antibiotics were causing the cancer.

To determine whether antibiotics cause breast cancer, investigators might choose two large groups of women who are similar in terms of age, education, body weight and many other characteristics and give them two kinds of pills one, of which is an antibiotic and one which contains only sugar for a given period of time. If the women receiving the antibiotic got more breast cancer than those getting the sugar pills, this would be strong evidence that the antibiotics cause cancer. But in this study the doctor would be deliberately exposing women to what could be a very harmful substance, the antibiotic that might cause breast cancer. We often have to make do with other kinds of studies that can establish associations but not causes.



Some Risk Factors for Breast Cancer

Risk factor
Relative risk

Having certain genes


Breast cancer in mother or sister


Alcohol (2-5 drinks/day)






Hormone replacement


5 or more children




What Are Other Risk Factors for Breast Cancer?

The chart above shows some of the other risk factors for developing breast cancer. Heredity is one factor. Some people have certain genes that increase their chances of getting breast cancer. The chart states that people with these genes have five times the normal risk of getting breast cancer, but most women who develop breast cancer do not have these genes. Another risk factor for developing breast cancer is a close relative who has been diagnosed with the disease. Note that abstaining from heavy drinking, exercise, and weight loss appear to reduce the risk of breast cancer. Clearly even if antibiotic usage is shown to be a contributing factor to causing breast cancer, it would be only one of many factors that influence one’s chances of getting this disease.

The Importance of Screening

Even if women have never been exposed to antibiotics and have no obvious risk factors, they should get yearly mammograms and clinical breast exams every year after age 40. This is the recommendation of a special government task force that was convened to make recommendations to the public. It is important to note that screening of minorities (African American and Hispanic women) still lags far behind that of Caucasian women, and this makes the breast cancer more likely to be fatal in these groups.

What Should I Do in Response to This Report?

The possibility - and it is important to stress that it is only a possibility - that the use of antibiotics might cause breast cancer is worrisome because antibiotics are such important medicines. They are among the most commonly prescribed drugs and their use has cured countless millions of people with diseases like tuberculosis, rheumatic fever, pneumonia, and many others. However as beneficial as they are, antibiotics are considerably overused. Patients often request and receive antibiotics from their doctors even when they have the symptoms of a common cold. Colds, which are caused by viruses, not bacteria, cannot be cured by antibiotics. Bacterial infections, which sometimes mimic the symptoms of a cold, can be cured with antibiotics.

The best policy is to avoid unnecessary antibiotics until we know more about the harmful effects (if any) of these drugs. If your doctor prescribes antibiotics you might want to mention your concern about their possible relationship to breast cancer just to make certain they are absolutely necessary in your case.

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